Generally, a high-efficient power amplifier used for a radio transmission apparatus such as a mobile base station has a strong nonlinear characteristic. Thus, when a modulated signal for high-speed radio communication is transmitted, the nonlinear distortion in such a power amplifier generates an out-of-band emission power in the modulated transmission signal, affecting an adjacent transmission channel.
A known system for suppressing the out-of-band emission of a power amplifier is the pre-distorter system in which an input signal is multiplied by a distortion signal having a reverse characteristic of the nonlinear characteristic of the power amplifier and before the input signal is provided to the power amplifier, to compensate for the nonlinear distortion. Particularly, with the adaptive pre-distorter system in which the distortion compensation is performed adaptively by feeding back the output of the power amplifier to the input side, the out-of-band emission can be suppressed significantly. Furthermore, a digital pre-distorter system has a look-up table storing a distortion compensation coefficient to be used for multiplication between the distortion signal with a reverse characteristics and the input signal. In the digital pre-distorter system, the output of the power amplifier is fed back to update the distortion compensation coefficient in the lookup table adaptively, the circuit configuration for the distortion compensation can be simplified (for example, Japanese National Publication of International Patent Application No. 2002-522989).
FIG. 1 illustrates a conventional configuration of an adaptive pre-distorter type distortion compensation apparatus that uses a lookup table.
In FIG. 1, an address generation unit 101 obtains an address value by calculating an input level from an orthogonal baseband input signal X(I,Q) input to the distortion compensation circuit, and reads out, from a lookup table (LUT) 102, a distortion compensation coefficient corresponding to the generated address value.
A multiplier 103 performs distortion compensation by multiplying input signal X(I,Q) by the distortion compensation coefficient that was read out from the LUT 102. The output of the multiplier 103 is converted into an analog signal in a D/A converter 104, and further, subjected to orthogonal modulation by a signal oscillated by a local oscillator (FW LOCAL) according to the transmission base station, in an orthogonal modulator (QMOD) 105.
The analog transmission signal that has been modulated is subjected to power amplification in a power amplifier 107, and its output is supplied to a transmission antenna that is not particularly illustrated in the drawing through a coupler (DC) 108, and transmitted from the antenna. The output of the power amplifier 107 is fed back via the DC 108 to the input side.
First, the output of the DC 108 is down-converted by a down converter (MIX) 109 using a signal oscillated from a local oscillator (FW LOCAL) 110 according to the transmission base station, and after conversion into a digital signal by an A/D converter 111, the signal is further converted into the baseband signal by a demodulator (DEM) 112.
For the feedback signal obtained as a result, the error with the input signal X(I,Q) that has been delayed by a delay circuit 114 is calculated by a subtracter 113, and the distortion compensation coefficient in the LUT 102 is updated by a Least Mean Square calculation circuit (LMS) 115 so as to minimize the error.
A central processing unit (CPU) 116 controls the update operation of the distortion compensation coefficient in the LUT 102, and the like.
The distortion compensation coefficient is gradually converged into a certain value and the converged distortion compensation coefficient is multiplied by the input signal X(I,Q) in the multiplier 103. Thus, in the steady state, the nonlinear distortion characteristic of the analog circuit portion is suppressed with good accuracy while maintaining high power efficiency.
As described above, the gain variation in the analog circuit portion of the forward (FW) system can be compensated by the amplitude value of the distortion compensation coefficient held in the LUT 102. Then, even when the gain of the FW system varies due to an influence from temperature or frequency, the analog gain variation amount is detected by the feedback signal and the value of the distortion compensation coefficient is updated in the direction to compensate for the variation amount, making it possible to correct the gain variation amount due to the temperature or frequency characteristics and the like of the analog circuit portion, at the same time with the distortion compensation.
However, since the gain value that can be compensated by the distortion compensation coefficient is limited, when the variation amount of the gain in the analog circuit due to temperature or frequency is large, it exceeds the range of the correction available by the distortion compensation coefficient. There has been a problem that when the variation amount exceeds the correction range, not only the distortion-compensation operation capability that is the original purpose deteriorates, but also the transmission output level becomes abnormal.
Therefore, there has been a conventional art as illustrated in FIG. 2 in which a variable attenuator (VATT) 201 for compensating for the gain variation of the FW system is disposed in the analogue circuit, and the variation amount is suppressed by performing the gain setting according to the temperature or frequency.
However, in this conventional art, a table for correcting the temperature and/or frequency is required. With the correction table reference configuration, the conventional art has problems such as that it is susceptible to the circuit variability; a non-volatile memory 204 for storing the correction value is required; and a temperature monitoring circuit 203 is required.
A method of adjusting the gain of a transmission signal in accordance with the amount of the distortion power of the transmission signal has been proposed (for example, Japanese Laid-open Patent Publication No. 2006-270797).
A system in which the amplitude of the distortion compensation coefficient is corrected in advance so that the transmission signal after the distortion compensation does not exceed the dynamic range of the D/A converter while maintaining the phase of the coefficient has been proposed (for example, Japanese Laid-open Patent Publication No. 2001-251148).
A method of adjusting the gain of a transmission signal so that the dynamic range of the D/A converter can be used to the maximum has been proposed (for example, Japanese Laid-open Patent Publication No. 2004-32252).